Acoustic wave devices using piezoelectric thin film resonators have been used as filters and duplexers of wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
The rapid diffusion of wireless systems has promoted the use of many frequency bands. Thus, filters or duplexers with steeper skirt characteristics have been desired. One way of making the skirt characteristic steeper is to increase the Q-value of the piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (Patent Document 1) discloses a piezoelectric thin film resonator including an annulus on a surface of one of the upper electrode and the lower electrode. Japanese Patent Application Publication No. 2014-161001 (Patent Document 2) discloses a piezoelectric thin film resonator including an insertion film inserted into the piezoelectric film in the outer peripheral region of the resonance region. U.S. Pat. No. 9,048,812 (Patent Document 3) discloses a piezoelectric thin film resonator including an annulus called a bridge in the piezoelectric film.
The piezoelectric thin film resonators of Patent Documents 1 through 3 can reduce the leak of the acoustic wave energy from the resonance region and improve the Q-value. However, since the annulus or the insertion film is located in the resonance region, the characteristics including the electromechanical coupling coefficient deteriorate.